Semiconductor memory device having cell array divided into a plurality of cell blocks

ABSTRACT

A semiconductor memory device includes a cell array having a plurality of memory cells grouped into a plurality of cell blocks and arranged in a matrix form, a plurality of word lines, a plurality of bit lines, bit line sense amplifiers (S/A), a cell block selection circuit,a plurality of data I/O lines, row decoders, a plurality of column selection signal lines, column decoders and a data buffer circuit. The data buffer circuit includes a first precharge circuit, connected to the data I/O lines, for precharging the data I/O lines to the same potential as a precharge potential of the bit lines, a second precharge circuit, connected to the data I/O lines, for precharging the data I/O lines to a potential different from the precharge potential of the bit lines, and selective drive circuit for generating control signals to be supplied to the first and second precharge circuit, and selectively driving the first and second precharge circuits to sense the data read out to the data I/O lines on the basis of the control signals.

This application is a Continuation of application Ser. No. 08/959,466 Filed on Oct. 28, 1997, now U.S. Pat. No. 5,862,090 which is a continuation of U.S. Ser. No. 07/944,729 filed Sep. 19, 1992 is now U.S. Pat. No. 5,734,619 which is a Continuation of U.S. Ser. No. 07/608,732 filed Nov. 5, 1990, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device such as a high integrated DRAM having a cell array divided into a plurality of cell blocks, wherein cell block selection is performed.

2. Description of the Related Art

In order to form a highly integrated DRAM, micro-patterning of elements and lines must be realized, and a cell array and a cell array drive circuit must be efficiently laid out within a predetermined chip area. In addition, the reading rate of cell data must be increased by decreasing a ratio C_(B) /C_(S) of the capacity C_(S) of a memory cell to a capacity C_(B) of bit lines. Furthermore, in order to reduce the power consumption due to charge/discharge of bit lines, the cell array must be divided into a plurality of cell blocks in the bit line direction. A highly integrated DRAM using a divided bit line and common Y-decoder method is inevitably required to satisfy these demands.

In order to realize this method, for example, a cell array is divided into four or eight cell blocks in the bit line direction. Bit lines in the respective cell blocks are independent of each other. A data I/O line shared by two cell blocks is arranged between two adjacent cell blocks. The bit lines in a selected one of the two adjacent cell blocks are connected to a data I/O line through a block selection gate and a column selection gate. A column selection signal line for controlling a column selection gate is continuously formed by a metal wire on the cell array constituted by the plurality of divided cell blocks, and a column selection signal as an output from a Y-decoder (i.e., column decoder) is supplied to the column selection signal line. A method of sharing at least part of a sense amplifier for bit line between adjacent cell blocks, i.e., a common sense amplifier method, is normally employed.

In a bit line precharge system of such a DRAM, it is known that an effective measure to reduce the power consumption and increase the speed of a bit line sense operation is to precharge bit lines at (1/2)Vcc.

A data I/O line, however, is preferably precharged to Vcc for the following reasons. First, assuming that the I/O line is precharged to the same potential as that of the bit lines, i.e., (1/2)Vcc, when a memory cell of a selected cell block is re-stored, an electric potential on bit line tends to be pulled up to an I/O line potential. A bit line sense amplifier is normally constituted by an NMOS sense amplifier and a PMOS sense amplifier. The NMOS sense amplifier is used to amplify a small signal. The PMOS sense amplifier is used to raise the potential of an "H"-level bit line to Vcc.

For this reason, the drive power of the PMOS sense amplifier is not originally set to be large. Therefore, when a bit line is electrically connected to an I/O line potential, since the electric potential on bit line is pulled up to the I/O line, the PMOS sense amplifier cannot satisfactorily raise the "H"-level bit line potential to Vcc. This phenomenon becomes conspicuous especially when an I/O line has a large capacity, and abnormal operations may be caused. Second, if an I/O line can be precharged to Vcc, the initial sensing time of the I/O line of circuit of a bit line sense amplifier can be shortened.

The conventional DRAM of the divided bit line and common Y-decoder method, however, does not employ the precharge method, in which bit lines are precharged to (1/2)Vcc and an I/O lines are precharged to Vcc, for the following reason. Each column selection signal line is continuously arranged across a plurality of cell blocks and is connected to the column selection gates of the respective cell blocks. With this arrangement, when a given column selection signal is selected, the column selection gates of non-selected cell blocks which data should be not read out are opened. Subsequently, bit line sense amplifiers located outside the selection gates of the non-selected cell blocks and precharged to (1/2)Vcc are connected to I/O lines precharged to Vcc. As a result, the precharge potentials of the bit line sense amplifier is subjected to breakdown. The precharge method has not been employed for the abovedescribed reason.

As described above, in the conventional DRAM of the divided bit line and common Y-decoder method, the precharge potential of each bit line cannot be set to be (1/2)Vcc and the precharge potential of each I/O line cannot be set to be Vcc. This interferes with realization of a further reduction in power consumption and an increase in operation speed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a DRAM of a divided bit line and common Y-decoder method, which can employ a precharge method in which each bit line is precharged to (1/2)Vcc and each I/O line is precharged to Vcc, so as to realize an increase in operation speed and a reduction in power consumption.

The characteristic feature is that DRAM formed in accordance with the present invention comprises:

a cell array including

a plurality of memory cells grouped into a plurality of cell blocks and arranged in a matrix form,

a plurality of word lines, arranged in each of the cell blocks in a column direction, for driving the memory cells in the column direction,

a plurality of bit lines, arranged to cross the word lines, for transferring data to/from the selected memory cells, and

bit line sense amplifiers respectively connected to the bit lines;

a cell block selection circuit for selecting one of the plurality of cell blocks of the cell array on active cycle;

a plurality of data I/O lines to which the bit lines in the cell block selected by the cell block selection circuit are connected through column selection gates;

row decoders for selectively driving the word lines;

a plurality of column selection signal lines arranged across the plurality of cell blocks of the cell array and connected to the column selection gates of the cell blocks;

column decoders for transferring column selection signals to the column selection signal lines; and

a data buffer circuit, connected to the respective data I/O lines, for sensing data read out to the respective data I/O lines, the data buffer circuit including

a first precharge circuit, connected to the data I/O lines, for precharging the data I/O lines at the same potential as a precharge potential of the bit lines,

a second precharge circuit, connected to the data I/O lines, for precharging the data I/O lines at a potential different from the precharge potential of the bit lines, and

a selective drive circuit for selecting one of the first and second precharge circuits and driving the selected circuit.

In addition, a DRAM of the present invention comprises:

a cell array including

a plurality of memory cells grouped into a plurality of cell blocks and arranged in a matrix form,

a plurality of word lines, arranged in each of the cell blocks in a column direction, for driving the memory cells in the column direction,

a plurality of bit lines, arranged to cross the word lines, for transferring data to/from the selected memory cells, and

bit line sense amplifiers respectively connected to the bit lines;

a cell block selection circuit for selecting one of the plurality of cell blocks of the cell array on active cycle;

a plurality of data I/O lines to which the bit lines in the cell block selected by the cell block selection circuit are connected through column selection gates;

row decoders for selectively driving the word lines;

a plurality of column selection signal lines arranged across the plurality of cell blocks of the cell array and connected to the column selection gates of the cell blocks;

column decoders for transferring column selection signals to the column selection signal lines; and

a selection gate control circuit which is arranged between the column selection signal lines and the selection gates and is controlled by a control signal from the cell block selection circuit to transfer the column selection signal to the selected column selection gate.

According to the present invention, with regard to bit lines and data I/O lines connected to each other on active cycle, the following two states can be obtained: a state wherein the precharge potential of the bit lines is (1/2)Vcc, and a state wherein the precharge potential of the data I/O lines is Vcc.

According to the first aspect of the present invention, all the data I/O lines on precharge cycle and non-selected data I/O lines on active cycle are set at the same precharge potential as that of the bit lines, i.e., (1/2)Vcc. In addition, only data I/O lines selected on active cycle are selectively precharged to Vcc.

According to the second aspect of the present invention, the precharge potential of all the data I/O lines is set at Vcc, and only the column selection blocks of a cell block selected on active cycle can be opened. With this operation, in the selected cell block, the data I/O lines precharged to Vcc are connected to the bit lines precharge at (1/2)Vcc.

In either of the first and second aspects, therefore, a relationship in potential between non-selected cell block areas is not broken. In addition, since the precharge potential of each bit line from which data is read out is set to be (1/2)Vcc and the precharge potential of a data I/O line is connected thereto is set to be Vcc, a high-speed data read operation can be performed.

As described above, according to the present invention, both the method of precharging bit lines to (1/2)Vcc and the method of precharging data I/O lines to Vcc can be used. Therefore, there is provided a highly integrated DRAM which can realize a reduction in power consumption and in chip size without decreasing the operation speed.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

FIG. 1 is a schematic diagram showing a chip layout of a DRAM of a divided bit line and common Y-decoder method according to the first embodiment of the present invention;

FIG. 2 is a circuit diagram showing a one-column portion of a typical subcell array in the first embodiment;

FIG. 3 is a circuit diagram showing one cell block of the subcell array;

FIG. 4 is a circuit diagram showing a data I/O buffer section connected to data I/O lines;

FIG. 5 is a timing chart for explaining an operation of the DRAM according to the first embodiment;

FIG. 6 is a circuit diagram showing a one-column portion of a subcell array of a DRAM according to the second embodiment of the present invention;

FIG. 7 is a circuit diagram showing a data I/O buffer section connected to data I/O lines of the DRAM;

FIG. 8 is a timing chart for explaining an operation of the DRAM of the second embodiment; and

FIG. 9 is a circuit diagram showing a one-column portion of a subcell array of a DRAM according to the third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A DRAM according to the first embodiment of the present invention will be described below with reference to FIGS. 1 to 5.

FIG. 1 shows a schematic chip layout of a DRAM of a divided bit line and common Y-decoder method, in which a cell array is divided into eight cell blocks. FIG. 2 shows an arrangement of a one-column portion of adjacent four cell blocks in FIG. 1. FIG. 3 shows an arrangement of one cell block in FIG. 2.

As shown in FIG. 1, a cell array arranged on a DRAM chip 1 is divided into eight cell blocks CA₀ to CA₇ in a bit line direction. These cell blocks CA₀ to CA₇ are grouped into two areas, i.e., an area including the cell blocks CA₀ to CA₃, and an area including the cell blocks CA₄ to CA₇. Row decoders 4₁ and 4₂ for selectively driving word lines are respectively arranged at end portions of the area the cell blocks CA₀ to CA₃ and of the area including the cell blocks CA₄ to CA₇. Column selection signal lines CSL_(i0) (i=1, 2, . . . , n) are continuously arranged on the four cell blocks CA₀ to CA₃ on the left side. Column selection signal lines CSL_(i1) (i=1, 2, . . . , n) are continuously arranged on the four cell blocks CA₀ to CA₃ on the left side. Column selection signal lines CSL_(i1) (i=1, 2, . . . , n) are continuously arranged on the four cell blocks CA₄ to CA₇ on the right side. Column decoders 5₁ and 5₂ for performing column selection are arranged at end portions of these column selection signal lines CSL_(i0) and CSL_(i1). More specifically, one column decoder 5₁ is shared by the four cell blocks CA₀ to CA₃ on the left side, whereas the other column decoder 5₂ is shared by the four cell blocks CA₄ to CA₇ on the right side.

In this DRAM, a bit line sense amplifier S/A and a data I/O line are arranged between the cell blocks CA₀ and CA₁. The bit line sense amplifier S/A and the data I/O line are shared by these cell blocks. Similarly, bit lines and data I/O lines are respectively arranged between the cell blocks CA₂ and CA₃, between the cell blocks CA₄ and CA₅, and between CA₆ and CA₇. The I/O lines on the left side are connected to a data line 3₁ through I/O buffer sections 2₁₁ and 2₁₂, respectively. The I/O lines on the right side are connected to a data line 3₂ through I/O buffer sections 2₂₁ and 2₂₂, respectively. These data lines 3₁ and 3₂ are connected to an external terminal through an input-output circuit 8.

A peripheral circuit 7 including an address buffer, an RAS control circuit, a CAS control circuit, and the like, and a cell block selector circuit 6 to be controlled by the peripheral circuit 7 are arranged between the two column decoders 5₁ and 5₂. The cell block selector circuit 6 has a function of selecting one cell block on active cycle.

FIG. 2 shows a cell array in more detail. FIG. 2 shows a detailed arrangement of a one-column portion of a subcell array 10 constituted by the four cell blocks CA₀ to CA₃ on the left side. FIG. 3 shows a detailed arrangement of the one cell block CA₀ in FIG. 2. In this embodiment, a cell array has a folded bit line structure.

A description will be made below in consideration of the cell block CA₀. As shown in FIG. 3, a large number of memory cells MC₁, MC₂, . . . each having a one-transistor/one-capacitor structure are respectively connected to a pair of bit lines BL₀ and BL₀ . Dummy cells DC₁ and DC₂ are respectively connected to the bit lines BL₀ and BL₀ . Word lines WL₁, WL₂, . . . for selectively driving the memory cells MC₁, MC₂, . . . , and dummy word lines DWL₁ and DWL₂ for selectively driving the dummy cells DC₁ and DC₂ are arranged to cross the bit lines BL₀ and BL₀ . These word lines and dummy word lines are designed to simultaneously drive memory cells and dummy cells corresponding to other columns.

A description will be made below in consideration of the cell blocks CA₀ and CA₁. Each bit line sense amplifier S/A comprises a PMOS sense amplifier PSA constituted by a PMOS flip-flop, and an NMOS sense amplifier NSA constituted by an NMOS flip-flop. Of these sense amplifiers, the PMOS sense amplifier PSA is arranged in each of the cell blocks CA₀ and CA₁. The NMOS sense amplifier NSA is arranged outside the cell blocks CA₀ and CA₁ so as to be shared by the two adjacent cell blocks CA₀ and CA₁. Block selection gates Q₅ and Q₆ to be controlled by a block selection signal BSL₁ are respectively arranged between the internal bit lines BL₀ and BL₀ in the cell block CA₀, and between external bit lines BL₀₁ and BL₀₁ which the common NMOX sense amplifier NSA is arranged. Similarly, block selection gates Q₇ and Q₈ to be controlled by a block selection signal BSL₀ are respectively connected between the cell block CA₁ and the external bit lines BL₀₁ and BL₀₁.

A bit line equalization circuit EQ shared by the two cell blocks CA₀ and CA₁ is arranged between the bit lines BL₀₁ and BL₀₁ on the right side of the common NMOS sense amplifier NSA. The bit line equalization circuit EQ comprises an equalization MOS transistor Q₃₁ for short-circuiting the bit lines BL₀₁ and BL₀₁ as a pair, and precharge MOS transistors Q₃₂ and Q₃₃ for setting these bit lines BL₀₁ and BL₀₁ to a precharge potential (1/2)Vcc. The bit lines BL₀₁ and BL₀₁ having the common NMOS sense amplifier NSA arranged therebetween are respectively connected to data I/O lines I/O and I/O through the column selection gates Q₁ and Q₂. The cell blocks CA₂ and CA₃ adjacent to these cell blocks CA₀ and CA₁ have the same arrangement as described above.

The column selection signal lines CSL_(i0) for transmitting column selection signals from the column decoder are arranged across the area of the four cell blocks CA₀ to CA₃. The column selection signals CSL_(i0) are connected to the column selection gates Q₁, Q₂, Q₃, and Q₄ of the four cell blocks CA₀ to CA₃ so as to simultaneously drive these column selection gates.

FIG. 4 shows a detailed arrangement of an I/O buffer section connected to the above-described subcell array 10. With regard to the cell blocks CA₀ and CA₁, the I/O buffer section includes an I/O buffer 16₁₁ and an I/O buffer control circuit 14₁. The I/O buffer 16₁₁ having a sense amplifier ill is connected to a pair of I/O lines I/O₀₁ and I/O₀₁ commonly provided for the adjacent cell blocks CA₀ and CA₁. This I/O buffer 16₁₁ includes a first I/O line precharge circuit 13₁ for precharging the I/O lines I/O₀₁ and I/O₀₁ to the same potential (1/2)Vcc as that of the bit lines, and a second I/O precharge circuit 12₁ for precharging the I/O lines I/O₀₁ and I/O₀₁ to the potential Vcc higher than (1/2)Vcc.

The first precharge circuit 13₁ comprises an equalization n-channel MOS transistor Q₁₅ for short-circuiting the I/O lines I/O₀₁ and I/O₀₁ , and precharge n-channel MOS transistors Q₁₃ and Q₁₄ for setting the I/O lines I/O₀₁ and I/C₀₁ to (1/2)Vcc. The second precharge circuit 121 comprises an equalization p-channel MOS transistor Q₁₈ and precharge p-channel MOS transistors Q₁₆ and Q₁₇.

The I/O buffer control circuit 14₁ selectively drives the I/O buffer 16₁₁ by using a precharge control signal CEQ and a sense control signal QSE.

An I/O buffer 16₁₂ including an I/O sense amplifier 11₂ is connected to a pair of I/O lines I/O₂₃ and I/O₂₃ commonly provided for other two cell blocks CA₂ and CA₃ in the same manner as described above. The I/O buffer 16₁₂ includes first and second I/O precharge circuits 13₂ and 12₂. An I/O buffer control circuit 14₂ is provided for the I/O buffer 16₁₂.

The cell block selection signals BSL₀ and BSL₁ are input to the I/O buffer control circuit 14₁ as control signals. Cell block selection signals BSL₂ and BSL₃ are input to the I/O buffer control circuit 14₂ as control signals. With this arrangement, when the cell block CA₀ or CA₁ is selected, i.e., the signal BSL₀ is BSL₁ is set at "L" level, a control signal IOS01 is set at "L" level, and the I/O sense amplifier 11₁ in the I/O buffer 16₁₁ is activated. At this time, in the I/O buffer 16₁₁, the control signal IOS₀₁ sets the first I/O line precharge circuit 13₁ for (1/2)Vcc precharge in an inactive state, and the control signal CEQ₀₁ sets the second I/O line precharge circuit 12₁ for Vcc precharge in an active state. At this time, since an "H"-level control signal IOS₂₃ is supplied to the I/O sense amplifier 11₂ in the other I/O buffer 16₁₂, the I/O sense amplifier 112 is not activated. The control signal IOS₂₃ sets the first I/O line precharge circuit 13₂ for (1/2)Vcc precharge in an active state, and the control signal CEQ₂₃ sets the second I/O line precharge circuit 12₂ for Vcc precharge in an inactive state. If the cell block CA₂ or CA₃ is selected, the above-described relationship is reversed. These I/O buffers are connected to the input-output circuit 8 through read/write data lines 3.

An operation of the DRAM of the divided bit line and common-Y decoder method having the above-described arrangement will be described with reference to a timing chart in FIG. 5, in which the cell block CA₀ of the eight cell blocks CA₀ to CA₇ is selected as indicated by a hatched portion in FIG. 1, and the data I/O line I/O₀₁ and I/O₀₁ are activated. Note that FIG. 5 shows the operation waveforms of the respective signals when the subcell array 10 consisting of the four cell block CA₀ to CA₃ is taken into consideration.

The block selection signals BSL₀ to BSL₃ are at "H" level (i.e., Vcc) before they are set on active cycle. Therefore, all the cell block selection gates Q₅ to Q₁₂ are in an ON state. When a row address is determined, for example, one block selection signal BSL₀ is set at "L" level. As a result, the block selection gates Q₇ and Q₈ are set in an OFF state, the cell block CA₁ of the two cell blocks CA₀ and CA₁ sharing the NMOS sense amplifier NSA is disconnected from the NMOS sense amplifier NSA. At the same time, the buffer control circuit 14₁ of the I/O buffer section 2₁₁ receives the "L"-level block selection signal BSL₀ and the "H"-level block selection signal BSL₁ . The I/O selection IOS₀₁ is set at "L" level by an AND gate G₁, so that the first precharge circuit 13₁ for (1/2)Vcc precharge in the I/O buffer 16₁₁ is not operated. In addition, the precharge control signal CEQ of "L" level and the I/O selection signal IOS₀₁ of "L" level are input to an OR gate G₂ so as to set the precharge control signal CEQ₀₁ at "L" level. As a result, the second precharge circuit 121 for Vcc precharge is activated. Furthermore, the potential of the sense signal QSE₀₁ is raised from (1/2)Vcc to Vcc by the I/O selection signal IOS₀₁ of "L" level and the sense control signal QSE of "L" level. Hence, the reference electric potential of the I/O sense amplifier 11₁ is increased to Vcc. With this operation, the I/O lines I/O₀₁ and I/O₀₁ which have been precharged to (1/2)Vcc are precharged to Vcc.

Meanwhile, a word line selected by the row decoder 4₁ is activated so that data of a memory cell arranged along the selected word line in the cell block CA₀ and of a dummy cell arranged along a selected dummy line are read out to the bit lines BL₀ and BL₀ . These data are transferred to the external bit lines BL₀₁ and BL₀₁ through block selection gates Q₅ and Q₆ The NMOS sense amplifier NSA is activated, and the PMOS sense amplifier PSA is then activated. Subsequently, the bit lines BL₀₁ and BL₀₁ are respectively set at Vcc and 0. The column selection signal CSL_(i0) selected by the column decoder 5₁ is set at "H" level. As a result, the column selection gates Q₁ and Q₂ are set in an ON state. The data on the bit lines BL₀₁ and BL₀₁ are read out to the I/O lines I/O₀₁ and I/O₀₁ through these column selection gates Q₁ and Q₂. Since the precharge control signal CEQ is set at "H" level at the same time that the column selection signal CSL_(i0) is set at "HH" level, the second precharge circuit 12₁ is set in an inactive state. Subsequently, the sense control signal QSE₀₁ is set at "L" level so that the I/O sense amplifier 11₁ is activated and the I/O lines I/O₀₁ and I/O₀₁ are respectively set at Vcc and 0.

During this read operation, the block selection gates Q₇ and Q₈ of the other cell block CA₁ sharing the NMOS sense amplifier NSA with the selected cell block CA₀ is kept in an OFF state. That is, the cell block CA₁ is disconnected from the NMOS sense amplifier NSA. Although the block selection gates Q₉, Q₁₀, Q₁₁, and Q₁₂ of the other non-selected cell blocks CA₂ and CA₃ are in an ON state, their precharge potential states are free from breakdown. More specifically, while the cell block CA₀ is selected, both the block selection signals BSL₂ and BSL₃ are kept at "H" level, as shown in FIG. 5. Therefore, the control signal IOS₂₃ of the first precharge circuit 13₂ for (1/2)Vcc precharge is set at "H" level by the I/O buffer control circuit 14₂ of the I/O buffer section 2₁₂ of the I/O lines I/O₂₃ and I/O₂₃ of the non-selected cell blocks CA₂ and CA₃. Similarly, the control signal CEQ₂₃ of the second precharge circuit 12₂ for Vcc precharge is kept at "H" level, and an activation signal QSE₂₃ of the I/O sense amplifier 11₂ is kept at (1/2) Vcc. That is, the data I/O lines 1/O₂₃ and I/O₂₃ are kept at (1/2)Vcc. Therefore, the column selection gates Q₃ and Q₄ are set in an ON state by the same column selection signal CSL_(i0) . Since both the data I/O lines I/O₂₃ and I/O₂₃ are set at a precharge potential of (1/2)Vcc, no problems are posed even if the NMOS sense amplifier of the non-selected cell blocks CA₂ and CA₃ is connected to the data I/O lines I/O₂₃ and I/O₂₃ .

When the active cycle is completed and a precharge cycle is set, the I/O lines precharged to Vcc on active cycle are precharged to (1/2)Vcc again.

In the first embodiment, the precharge method is employed in the above-described manner, in which only I/O lines selected on active cycle are precharged to Vcc on the basis of the (1/2)Vcc precharge method. According to the DRAM of the first embodiment, therefore, both a reduction in power consumption and in chip area by the (1/2)Vcc precharge method and an increase in operation speed by the Vcc precharge method can be realized. Vcc precharge of a selected I/O line may be completed by the time that a column selection signal is activated. Therefore, a satisfactory margin can be set. That is, the method of this embodiment does not interfere with an increase in operation speed and does not require an especially large MOS transistor for Vcc precharge. The selected I/O line is precharged to (1/2)Vcc again when the active cycle is completed and a precharge cycle is set. This operation, however, is performed at the same time that other bit lines and I/O lines are precharged, and hence no extra time is required.

A DRAM of a divided bit line and common-Y decoder method according to the second embodiment of the present invention will be described below with reference to FIG. 6. The overall arrangement of the second embodiment is the same as that of the first embodiment described with reference to FIG. 1. FIG. 6 shows an arrangement of a one-column portion of the DRAM in correspondence with the arrangement shown in FIG. 2. The same reference numerals in FIG. 6 denote the same parts as in FIG. 6, and a detailed description thereof will be omitted.

In the first embodiment, the column selection signal lines CSL_(i0) arranged across a plurality of cell blocks are directly connected to a plurality of column selection gates so as to simultaneously open/close the column selection gates. In order to perform Vcc precharge of only selected I/O lines of a plurality of data I/O lines, Vcc precharge and (1/2)Vcc precharge circuits for selectively driving data I/O lines are arranged. In contrast to this, the second embodiment includes selective drive circuits 21a and 21b for selectively driving column selection gates Q₁ and Q₂, and Q₃ and Q₄, as shown in FIG. 6. The selective drive circuit 21a is arranged between column selection signal lines CSLi0 arranged across a plurality of cell blocks and the column selection gates Q₁ and Q₂ to be driven thereby, whereas the selective drive circuit 21b is arranged between the column selection signal lines CSL_(i0) and the column selection gates Q₃ and Q₄.

In the second embodiment, each of the selective drive circuits 21a and 21b is constituted by a two-input NAND gate G₁₁ and a two-input AND gate G₁₂. The NAND gate of one selective drive circuit 21a receives two block selection signals BSL₀ and BSL₁ . The NAND gate of the other selective drive circuit 21b receives remaining two block selection signals BSL₂ and BSL₃ . One input terminal of each AND gate G₁₂ is connected to a corresponding column selection signal line CSL_(i0), and the other input terminal is connected to the output of a corresponding one of the NAND gates G₁₁. An output from the AND gate G₁₂ of one selective drive circuit 21a is used as a control signal for the column selection gates Q₁ and Q₂ arranged between the cell blocks CA₀ and CA₁. An output from the AND gate G₁₂ of the other selective drive circuit 21b is used as a control signal for the column selection gates Q₃ and Q₄.

Two pairs of data I/O lines I/O₀₁ and I/O₀₁ , and I/O₂₃ and I/O₂₃ to which the bit lines of this subcell array 10 are connected are connected to an I/O line sense amplifier 22 and an I/O line precharge circuit 23 for Vcc precharge, as shown in FIG. 7.

In the second embodiment having the above-described arrangement, when one of the column selection signal lines CSL_(i0) is selected on active cycle, all the column selection gates Q₁ and Q₂, and Q₃ and Q₄ are not simultaneously set in an ON state. A signal from the column selection signal line CSL_(i0) is selectively supplied to the column selection gates Q₁ and Q₂ or Q₃ and Q₄ in accordance with selection of a cell block.

An operation of the DRAM of this embodiment will be described in detail below with reference to FIG. 8. Similar to the description of the first embodiment, FIG. 8 shows the operation waveforms of the respective signals appearing when data is read out from the cell block CA₀. All the block selecting signals BSL₀ to BSL₃ are at "H" level before an active cycle is set. Therefore, all the cell block selection gates Q₅ to Q₁₂ are in an ON state. When a row address is determined, for example, the block selection signal BSL₀ is set at "L" level. As a result, the block selection gates Q₇ and Q₈ are set in an OFF state. That is, the cell block CA₁ of the two cell blocks CA₀ and CA₁ sharing the NMOS sense amplifier NSA is disconnected from the NMOS sense amplifier NSA.

A word line WL selected by a row decoder is activated, and data of a selected memory cell in the cell block CA₀ and of a dummy cell are read out to the bit lines BL₀ and BL₀ . These data are transferred to external bit lines BL₀₁ and BL₀₁ through the block selection gates Q₅ and Q6. The NMOS sense amplifier NSA is activated, and the PMOS sense amplifier PSA is then activated. As a result, the bit lines BL₀₁ and BL₀₁ are respectively set at Vcc and 0. Subsequently, the column selection signal CSL_(i0) selected by a column decoder 5₁ is set at "H" level. Since the block selection signals BSL₀ and BSL₁ are respectively set at "H" level and "L" level at this time, an output control signal CSL_(i0a) from the selective drive circuit 21a is set at "H" level. As a result, the column selection gates Q₁ and Q₂ are set in an ON state. Consequently, the data on the bit lines BL₀₁ and BL₀₁ are read out to the I/O lines I/O₀₁ and I/O₀₁ through the column selection gates Q₁ and Q₂, respectively.

As described above, during this read operation, the block selection gates Q₇ and Q₈ of the cell block CA₁, which share the NMOS sense amplifier NSA with the cell block CA₀, are kept in an OFF state. That is, the cell block CA₁ is disconnected from the NMOS sense amplifier NSA. On the other hand, the block selection gates Q₉, Q₁₀, Q₁₁, and Q₁₂ of the other non-selected cell blocks CA₂ and CA₃ are in an ON state. However, the precharge potentials of these portions are free from breakdown for the following reason. While the cell block CA₀ is selected, both the block selection signals BSL₂ and BSL₃ are kept at "H" level, as shown in FIG. 8. Thus, a control signal CSL_(iob) obtained from the selective drive circuit 21b is kept at "L" level, and the column selection gates Q₃ and Q₄ of the cell blocks CA₂ and CA₃ are kept in an OFF state. Therefore, the bit lines BL₂₃ and BL₂₃ precharged to (1/2)Vcc are not connected to the data I/O lines I/O₂₃ and I/O₂₃ precharged to Vcc. As shown in FIG. 8, during this read operation, the bit lines BL₂₃ and BL₂₃ are kept at (1/2)Vcc, whereas the data I/O lines I/O₂₃ and I/O₂₃ are kept at Vcc.

As described above, in the second embodiment, (1/2)Vcc precharge of bit lines and Vcc precharge of I/O lines are simultaneously performed.

A DRAM according to the third embodiment of the present invention will be described below with reference to FIG. 9. In the third embodiment, the selective drive circuits 21a and 21b in FIG. 6 are modified. More specifically, a circuit portion corresponding to the AND gate G₁₂ in each of the selective drive circuits 21a and 21b is constituted by an inverter I, a transfer gate, and an n-channel MOS transistor Q43 for short-circuit. The transfer gate consists of an n-channel MOS transistor Q41 and a p-channel MOS transistor Q42. Other arrangements are the same as those in FIG. 6.

An operation of the third embodiment is the same as that of the second embodiment. Similar to the second embodiment, assume that block selection signals BSL₀ to BSL₃ are set in such a manner that the BSL₀ ="L" level and BSL₁ =BSL₂ =BSL₃ ="H" on active cycle. In this case, the output of a NAND gate Gll of one selective drive circuit 21a is at "H" level. Therefore, both the MOS transistors Q₄₁ and Q₄₂ are in an ON state, and the MOS transistor Q₄₃ is in an OFF state. A signal of "H"level from a column selection signal line CSL_(i0) is transferred to column selection gates Q₁ and Q₂ through the selective drive circuit 21a. The output of the NAND gate G₁₁ of the other selective drive circuit 21b is set at "L" level. Consequently, the MOS transistors Q₄₁ and Q₄₂ are in an OFF state, and the MOS transistor Q₄₃ is in an OFF state. That is, the "H"-level signal from the column selection signal CSL_(i0) is not transferred to the column selection gates Q₃ and Q₄.

As described above, since column selection gates connected to non-selected cell blocks are not rendered conductive, (1/2)Vcc precharge of bit lines and Vcc precharge of data I/O lines can be simultaneously performed.

In the third embodiment, the number of elements used for each of the selective drive circuits 21a and 21b is smaller than that in the second embodiment described with reference to FIG. 6. More specifically, in the second embodiment, the AND gate G₁₂ is normally constituted by six elements. In contrast to this, a portion corresponding to the AND gate G₁₂ in the third embodiment can be constituted by five elements, namely the three MOS transistors Q₄₁ to Q₄₃, and two transistors constituting the inverter I. Since the selective drive circuits 21a and 21b must be arranged for each column selection signal line, even a decrease in number of elements by one can greatly contribute a reduction in DRAM chip area.

The prevent invention is not limited to the above embodiments. For example, in the embodiments, a column decoder is located in substantially the center of a DRAM chip. However, such an layout can be changed as needed.

In addition, if NAND gates in selective drive circuits are arranged outside a cell array, the DRAM of the present invention can be integrated at a high density. Furthermore, in the embodiments, a bit line precharge potential is set to be (1/2)Vcc, and a data I/O line precharge potential is set to be two or more values such as Vcc and (1/2)Vcc in the first embodiment and to be Vcc only in the second and third embodiments. However, proper precharge potentials other than these values can be selected.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices, and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A dynamic random access memory, comprising:first and second memory cell arrays, each having a plurality of memory cells with each of the plurality of memory cells being connected to bit lines; a sense amplifier commonly provided for said first and second memory cell arrays; a first precharge circuit, commonly provided for said first and second memory cell arrays and driven by a first control signal; a column select gate transmitting data from/to said plurality of memory cells provided in said first and second memory cell arrays; a pair of data lines through which said data is transferred; a second precharge circuit connected to said pair of data lines and driven by a second control signal; and a third precharge circuit connected to said pair of data lines and driven by a third control signal.
 2. The dynamic random access memory according to claim 1, wherein said second precharge circuit precharges said pair of data lines to a lower potential level than said third precharge circuit does.
 3. The dynamic random access memory according to claim 1, wherein said second precharge circuit precharges said pair of data lines to a V_(cc) /2 level and said third precharge circuit precharges said pair of data lines to a V_(cc) level.
 4. A dynamic random access memory, comprising:first and second memory cell arrays, each having a plurality of memory cells with each plurality of memory cells being connected to bit lines; a sense namplifier commonly provided for said first and second memory cell arrays; a first precharge circuit commonly provided for said first and second memory cell arrays and driven by a first control signal; a column select gate transmitting data from/to said plurality of memory cells provided in said first and second memory cell arrays; a first pair of data lines through which said data is transferred; a second precharge circuit connected to said first pair of data lines and driven by a second control signal; a third precharge circuit connected to said first pair of data lines and driven by a third control signal; a second pair of data lines through which said data transmitted from said first pair of data lines is transferred; and a fourth precharge circuit connected to said second pair of data lines and driven by a fourth control signal, said fourth precharge circuit precharging said second pair of data lines.
 5. The dynamic random access memory according to claim 4, wherein said second precharge circuit precharges said first pair of data lines to a lower potential level than said third precharge circuit does.
 6. The dynamic random access memory according to claim 4, wherein said second precharge circuit precharges said first pair of data lines to a V_(cc) /2 level and said third precharge circuit precharges said first pair of data lines to a V_(cc) level.
 7. The dynamic random access memory according to claim 6, wherein said third and fourth precharge circuits precharge said first and second pairs of data lines to a same level.
 8. A dynamic random access memory, comprising:first and second memory cell arrays, each having a plurality of memory cells with each plurality of memory cells being connected to bit lines; a sense amplifier commonly provided for said first and second memory cell arrays; a first precharge circuit commonly provided for said first and second memory cell arrays and driven by a first control signal; a column select gate transmitting data from/to said plurality of memory cells provided in said first and second memory cell arrays; a first pair of data lines through which said data is transferred; a second precharge circuit connected to said first pair of data lines and driven by a second control signal, said second precharge circuit precharging said first pair of data lines to a first potential; a second pair of data lines through which data from said first pair of data lines is transferred; and a third precharge circuit, connected to said second pair of data lines and driven by a third control signal, said third precharge circuit precharging said second pair of data lines to a second potential.
 9. The dynamic random access memory according to claim 8, wherein said second precharge circuit precharges said first pair of data lines to a lower potential level than said third precharge circuit precharges said second pair of data lines.
 10. The dynamic random access memory according to claim 8, wherein said second precharge circuit precharges said first pair of data lines to a V_(cc) /2 level and said third precharge circuit precharges said second pair of data lines to a V_(cc) level. 